The current energy "crisis" or shortage has created a need for alternative sources of energy. One such source of energy that shows promise is the solar powered rotary actuator which, for example, may comprise a heat pump or air conditioner powered by the sun. In an exemplary system, power from photo-voltaic cells would be conditioned by a conventional inverter to drive an a.c. induction motor which, in turn, would drive the heat pump compressor. In another embodiment, steam generated from a solar collector system would drive a turbine which, in turn, would drive the compressor. In the more elaborate and expensive systems, excess energy is collected and stored by batteries or other storage devices to be used during those times when the amount of sunlight is insufficient to carry the load. In addition, a conventional a.c. induction motor, driven by power from the local utility, is required to drive the compressor during such periods of insufficient sun. In the past, a detector has been used to determine when the power produced by the sun is insufficient and to switch the utility driven motor into the system to drive the compressor. It will be appreciated that during a partly cloudy day, the auxiliary motor, under the control of the detector, would continually be switched on and off as the sun is blocked and unblocked by the clouds. Such operation is objectionable and requires the use of an excess of solar cells and storage batteries, thereby greatly increasing the cost of installation, and maintenance, of the system.